The nucleus accumbens is found in the ventral striatum and is composed of medium spiny neurons.[3][4] It is subdivided into limbic and motor subregions known as the shell and core.[2] The medium spiny neurons receive input from both the dopaminergic neurons of the VTA and the glutamatergic neurons of the hippocampus, amygdala, and medial prefrontal cortex. When they are activated by these inputs, the medium spiny neurons’ projections release GABA onto the ventral pallidum.[2] The release of dopamine in this structure drives the mesolimbic system.

Amygdala

The amygdala is a large nuclear mass in the temporal lobe anterior to the hippocampus. It has been associated with the assignment of emotions, especially fear and anxiety. There are two, one in each temporal lobe, and their functions may belateralized.

Hippocampus

The hippocampus is located in the medial portion of the temporal lobe. It is known for its association with double memory (i.e., bothprocedural and declarative memory).

There is some controversy regarding dopamine’s role in the reward system. Three hypotheses—hedonia, learning, and incentive salience—have been proposed as explanations for dopamine’s function in the reward system.[1] The hedonia hypothesis suggests thatdopamine in the nucleus accumbens acts as a ‘pleasureneurotransmitter‘. In the late 1970s, it was found that some drugs of abuse involved dopamine activity, in particular in the nucleus accumbens, to cause the “high” or euphoric state. However, not all rewards or pleasurable things involve activation of the reward system, which may suggest that the mesolimbic pathway may not be just a system that works merely off enjoyable things (hedonia).[5] Learning, on the other hand, deals with predictions of future rewards and association formation. Studies have shown that rats that had their ventral tegmental area and nucleus accumbens destroyed do not lose their learning capabilities, but rather lack the motivation to work for a reward.[1] Incentive salience (wanting) stands out as a possible role for dopamine, as it regards this molecule as being released when there is a stimulus worth working hard for, thus making an individual work to get it. This is one of the reasons that dopamine transport has been extensively studied in ADD and ADHD. It is now widely understood that most people suffering from some form of attention deficit disorder most likely lack dopamine stimulation. This also explains why dopamine reuptake inhibitors and stimulants often dramatically improve symptoms of attention disorders. In self-administration studies, animals have been trained to give an operant response (lever press, nose poke, wheel turn, etc.) in order to obtain either a drug or a mate. It has been shown that the animals will continue to perform the required task until the reward is received, or fatigue sets in.[2][5]

Since the mesolimbic pathway is shown to be associated with feelings of reward and desire, this pathway is heavily implicated inneurobiological theories of addiction, schizophrenia, and depression.[6][7][8] Drug addiction, the loss of control over drug use or the compulsive seeking and taking of drugs despite adverse consequences, with the four major classes of abused drugs (psychostimulants, opiates, ethanol, and nicotine) are due to increased dopamine transmission in the limbic system-each by different mechanisms.[2][9]Like drug addiction, schizophrenia and depression have similar structural changes with dopamine transmission.[6]